French Experience in Tube-to-Tubesheet Joint Technology for Steam Generators

Abstract

Various types of tube-to-tube sheet joints have been used on French nuclear steam generators. This paper presents these different joints with their main properties and related operating experience. The first joints to be used for a nuclear power plant were made by explosive expansion. This technology was applied on site and was intended to suppress the crevice originally designed between tube and tube sheet. Operating experience is described. For subsequent plants, it appeared advisable to use rolling in order to avoid the adverse effects of the crevice. However, with this process, high residual stresses occurred on the primary side (E 3001350 MPa) and secondary side (= 200 MPa). The main concern at that time was secondary side corrosion. Therefore kiss rolling was applied to decrease the residual secondary side stresses (( 100 MPa). Operating experience has shown that this objective was achieved, but due to the initial choice of material (Inconel 600 MA and later lnconel 600 TT), primary water stress corrosion was observed. Shot peening was used to prevent or delay cracking. To improve the behavior of this joint, a comprehensive study of different technologies was performed in the mid 80's, although the decision to select lnconel 690 had been taken in 1984 and PWSCC was not considered to be a potential problem for this alloy. This study demonstrated that, to limit residual stresses on the primary and secondary side, hydraulic expansion with optimized parameters was the best candidate. Special attention was paid to the tightness of this joint which was of excellent quality. Since its industrial implementation presents no difficulties, the hydraulic expansion process was selected for the French and export programs.